The spinal column of vertebrates provides support to bear weight and protection to the delicate spinal cord and spinal nerves. The spinal column comprises a series of vertebrae stacked on top of each other. There are typically seven cervical (neck), twelve thoracic (chest), and five lumbar (low back) segments. Each vertebra has a cylindrical shaped vertebral body in the anterior portion of the spine with an arch of bone to the posterior which covers the neural structures. Between each vertebral body is an intervertebral disk, a cartilaginous cushion to help absorb impact and dampen compressive forces on the spine. To the posterior the laminar arch covers the neural structures of the spinal cord and nerves for protection. At the junction of the arch and anterior vertebral body are articulations to allow movement of the spine.
Various types of problems can affect the structure and function of the spinal column. These can be based on degenerative conditions of the intervertebral disk or the articulating joints, traumatic disruption of the disk, bone or ligaments supporting the spine, tumor or infection. In addition congenital or acquired deformities can cause abnormal angulation or slippage of the spine. Slippage (spondylolisthesis) anterior of one vertebral body on another can cause compression of the spinal cord or nerves. Patients who suffer from one of more of these conditions often experience extreme and debilitating pain, and can sustain permanent neurologic damage if the conditions are not treated appropriately.
One technique of treating these disorders is known as surgical arthrodesis of the spine. This can be accomplished by removing the intervertebral disk and replacing it with bone and immobilizing the spine to allow the eventual fusion or growth of the bone across the disk space to connect the adjoining vertebral bodies together. The stabilization of the vertebra to allow fusion is often assisted by a surgically implanted device to hold the vertebral bodies in proper alignment and allow the bone to heal, much like placing a cast on a fractured bone. Such techniques have been effectively used to treat the above described conditions and in most cases are effective at reducing the patient's pain and preventing neurologic loss of function. However, there are disadvantages to the present stabilization devices.
The spinal fixation device needs to allow partial sharing of the weight of the vertebral bodies across the bone graft site. Bone will not heal if it is stress shielded from all weight bearing. The fixation device needs to allow for this weight sharing along with the micromotion that happens during weight sharing until the fusion is complete, often for a period of three to six months or longer, without breakage. The device must be strong enough to resist collapsing forces or abnormal angulation during the healing of the bone. Loss of alignment during the healing phase can cause a poor outcome for the patient. The device must be secure in its attachment to the spine to prevent migration of the implant or backout of the screws from the bone which could result in damage to the structures surrounding the spine, resulting in severe and potentially life threatening complications. The device must be safely and consistently implanted without damage to the patient.
Several types of anterior spinal fixation devises are in use currently. One technique involves placement of screws all the way through the vertebral body, called bicortical purchase. The screws are placed through a titanium plate but are not attached to the plate. This device is difficult to place, and over penetration of the screws can result in damage to the spinal cord. The screws can back out of the plate into the surrounding tissues as they do not fix to the plate. Several newer generation devices have used a unicortical purchase of the bone, and in some fashion locking the screw to the plate to provide stability and secure the screw from backout. Problems have resulted from over ridged fixation and stress shielding, resulting in nonunion of the bony fusion, chronic micromotion during healing resulting in stress fracture of the fixation device at either the screw or the plate, insecure locking of the screw to the plate resulting in screw backout, or inadequate fixation strength and resultant collapse of the graft and angulation of the spine.
These devices are often designed to support and bridge across a group of vertebrae, for example a group of three. Because these devices are typically bridged across the bone, for example in the cervical region, they occasionally aggravate the esophagus, making it difficult for one to swallow food. In addition, the screws are installed into the bone normal, i.e., 90° to the plate's surface. Local angularity in the vertebral column often causes high shearing stresses to be applied to the screws. These stresses may fatigue the screws or cause deformation of the screw holes.
Thus, there is a need for a device and method of supporting adjacent vertebrae that avoids these problems and risks to the patient.
Bone mechanical properties greatly influence the stiffness of vertebra-implant-vertebra constructs. Bone properties are a function of many factors including bone mineral density, age, and sex. For comparative purposes, it will be assumed that bone properties are constant in the following discussions. Preparation of the bone to receive the implant can influence strength and stiffness. Again, for comparative purposes, it will be assumed that bone preparation is not a variable except when specifically discussed.
Interbody devices are typically classified as threaded cylinders or screws (e.g., BAK C), boxes (usually tapered rectangular boxes with ridges like the Brantigan cage), or vertical cylinders (e.g., Harms cage). Threaded cylinders usually have small pores and graft material is located inside the hollow interior of the cylinder. Device stiffness might be an issue for such designs. Boxes and vertical cylinders are generally open structures and in these devices a combination of device stiffness and subsidence are responsible for loading the graft.
The stiffness of a material and the stiffness of the structure (device) are often confused. Material stiffness is quantified by Modulus of Elasticity, the slope of the stress-strain curve. Steel has a high modulus, and gold has a low modulus. Structural or device stiffness is a function of dimensions of the part and the material from which the part is made. For example, steel is a very stiff material. However, when formed into the shape of a structure like a paperclip it is easily bent. Stiffness of an assembly or construct can be influenced by connections. While a paperclip and even a piece of paper are strong in tension, when assembled with a piece of paper a paperclip can be easily pulled off because they are only held together by friction.
The same analogy holds for inter-vertebral implants. For instance, consider a simplified construct consisting of a bone block, an interbody device, and a bone block, stacked on top of each other and loaded in compression. If the device is made from a low modulus material but has a large footprint on the bone, and conforms very well to the bone, the assembly can be very stiff in compression. The slope of the load-deflection curve would be steep. A device made from a high modulus material that has a small footprint on the bone and sharp edges might simply punch into the bone under compressive load. The slope of the load-deflection curve would be low, making the construct appear very compliant despite the fact that the device is rigid.
The terms flexibility and stiffness are used in connection with both the range of motion of the spine and the mechanical performance of implant constructs, and the distinction is not always clearly defined.
The spinal column of vertebrates provides support to bear weight and protection to the delicate spinal cord and spinal nerves. The spinal column includes a series of vertebrae stacked on top of each other. There are typically seven cervical (neck), twelve thoracic (chest), and five lumbar (low back) segments. Each vertebra has a cylindrical shaped vertebral body in the anterior portion of the spine with an arch of bone to the posterior, which covers the neural structures. Between each vertebral body is an intervertebral disc, a cartilaginous cushion to help absorb impact and dampen compressive forces on the spine. To the posterior the laminar arch covers the neural structures of the spinal cord and nerves for protection. At the junction of the arch and posterior vertebral body are articulations to allow movement of the spine.
Various types of problems can affect the structure and function of the spinal column. These can be based on degenerative conditions of the intervertebral disc or the articulating joints, traumatic disruption of the disc, bone or ligaments supporting the spine, tumor or infection. In addition congenital or acquired deformities can cause abnormal angulation or slippage of the spine. Slippage (spondylolisthesis) anterior of one vertebral body on another can cause compression of the spinal cord or nerves. Patients who suffer from one of more of these conditions often experience extreme and debilitating pain, and can sustain permanent neurological damage if the conditions are not treated appropriately.
One technique of treating these disorders is known as surgical arthrodesis of the spine. This can be accomplished by removing the intervertebral disc and replacing it with bone and immobilizing the spine to allow the eventual fusion or growth of the bone material across the disc space to connect the adjoining vertebral bodies together. The stabilization of the vertebra to allow fusion is often assisted by a surgically implanted device to hold the vertebral bodies in proper alignment and allow the bone to heal, much like placing a cast on a fractured bone. Such techniques have been effectively used to treat the above-described conditions and in most cases are effective at reducing the patient's pain and preventing neurological loss of function. However, there are disadvantages to the present stabilization devices.
Several types of anterior spinal fixation devices are in use currently. One technique involves placement of screws all the way through the vertebral body, called bicortical purchase. The screws are placed through a titanium plate but are not attached to the plate. This device is difficult to place, and over penetration of the screws can result in damage to the spinal cord. The screws can back out of the plate into the surrounding tissues, as they do not fix to the plate. Several newer generation devices have used a unicortical purchase of the bone, and in some fashion locking the screw to the plate to provide stability and secure the screw from back out. Problems have resulted from over rigid fixation and stress shielding, resulting in nonunion of the bony fusion, chronic micro-motion during healing resulting in stress fracture of the fixation device at either the screw or the plate, insecure locking of the screw to the plate resulting in screw back out, or inadequate fixation strength and resultant collapse of the graft and angulation of the spine.
These devices are often designed to support and bridge across a group of vertebrae, for example a group of three. Because these devices are typically bridged across the bone, for example in the cervical region, they occasionally aggravate the esophagus, making it difficult for one to swallow food. In addition, the screws are installed into the bone normal, i.e., 90° to the plate's surface. Local angularity in the vertebral column often causes high shearing stresses to be applied to the screws. These stresses may fatigue the screws or cause deformation of the screw holes.